The major objective of tumor immunotherapy involves harnessing the power of the host's immune system to target and eliminate tumors. Antibodies targeting tumor antigens, such as anti-CD20, anti-Her2neu, and anti-EGFR, are currently used to treat a variety of malignancies. While these antibodies can kill malignant cells by immediate mechanisms, such ADCC mediated by NK cells and monocytes through activating Fc?Rs, evidence from both human and animal studies indicate that they also appear to induce a vaccinal effect that results in anti-tumor T cell responses and long-term protection. Further support for the importance of T cell-mediated responses has come from the use of agonistic antibodies directly targeting key TNF receptor (TNFR) superfamily members that control immune responses and have been demonstrated as an effective approach to elicit anti-tumor immunity in pre-clinical studies. However, as opposed to the activating Fc?R requirements for ADCC mediated killing and induction of a vaccinal effect, the anti-tumor activities of anti-CD40 antibodies, for example, have recently been found to require engagement of the inhibitory Fc?RIIb and can be enhanced by Fc?RIIb-targeted Fc engineering. We will focus on determining the mechanisms underlying the differential Fc?R requirements for eliciting anti-tumor T cell immune responses through activating Fc?R engagement on antigen presenting cells, and the inhibitory Fc?R-dependent mechanisms of the anti-tumor activities of agonistic anti-TNFR antibodies in vivo. In Aim 1, an established in vivo lymphoma model will be utilized to characterize the cellular and molecular mechanisms of a CD20 mAb-targeted vaccinal effect, including the required cell types and specific activating Fc?Rs. A novel humanized mouse strain that expresses human CD20 and Fc?Rs will be exploited to assess the relative contributions of huFc?Rs during anti-CD20 induced tumor clearance and vaccinal effect and to investigate methods by which the vaccinal effect may be enhanced. This study will also determine whether selectively enhancing huFc-huFc?R interactions or co-administration of various inflammatory stimuli and adjuvants augments the vaccinal effect in vivo. Aim 2 of this study will elucidate the mechanisms underlying the essential role of inhibitory Fc?R co-engagement during the anti-tumor activities of agonistic anti-TNFR antibodies. The impact of Fc?RIIb-mediated signaling to the in vivo activities of agonistic anti-TNFR antibodies will be determined and the expression profiles or levels of this inhibitory receptor as compared to activating Fc?Rs will be evaluated. The contribution of Fc?RIIb expression by different immune cells to the in vivo activities of different anti-TNFR antibodies (e.g., anti-CD40 and anti-DR5) will be determined in Fc?RIIb conditional knockout mice. Novel mouse strains with defective Fc?R signaling, or aberrant Fc?R expression levels or profiles will be used to assess the contribution of these factors during anti-TNFR antibody agonistic function.